Fiber optics has gained prominence in telecommunications, instrumentation, cable T.V., network, and data transmission and distribution. Major application of fiber optics has been in the area of telecommunications. Over the last decade there has been a significant change-over from wires and co-axial cables to optical fibers for telecommunication systems and information services. This anticipated change is dictated by the benefits of improved technology as well as economics. Increasing cost and demand for high data rate or large bandwidth per transmission channels and the lack of available space in already congested conduits in every metropolitan area are a key reason in the change over from the wires to fiber optics. Additionally, fiber optical devices interface well with digital data processing equipment, and their technology is compatible with modern microelectronic technology.
A key component of the modern optical fiber system is the photodetector. The function of the detector in a fiber optic communication link is to convert optical power into electrical response. The most common detector used in fiber applications is the photodiode, which acts as a converter of optical power to electrical current. The type of semiconductor photodiode commonly used for fiber optics application has a reverse bias p-n junction.
The two most commonly used photodetectors heretofore are the p-i-n photodiode (p/intrinsic/n type conductivity) and the avalanche photodiode (APD). Both types of photodiodes are instantaneous photon-to-electron converters where absorbed photons generate hole-electron pairs to produce an electric current. The p-i-n (or pin) and avalanche photodiodes are actually modified p-n junction devices with additional layers at differing doping levels to produce either more efficient quantum conversion or avalanche gain through ionization. Basically, a photon is absorbed in a relatively high E (electric) field region, where an electron-hole pair is created. This will produce current in the detector circuit. To obtain high quantum efficiency, the pin photodiode provides absorption in a relatively highly resistive central intrinsic region.
An alternative approach to the pin photodiode is one in which higher detector currents are created as a result of the avalanche gain effect, as in the case of the avalanche photodiode (APD). In this case, a single electron-hole-pair may generate hundreds of secondary electron-hole-pairs. Because these events occur at random and are of a statistical nature, the noise generated in these devices can be a limiting factor on detectivity. In designing a solid state photodiode, there is a tradeoff between quantum efficiency and the speed of response. For high quantum efficiency, the width of the intrinsic region of a pin device, for example, should be comparable to two or three times the absorption length of light in the material being used. For high speed response, the device should be as thin as possible to minimize carrier transit times. Silicon detectors operating at wavelengths below 0.9 micrometers are capable of both high quantum efficiency (in excess of 50%) and fast response (less than 0.5 nanoseconds (ns)). At longer wavelengths, however, the absorption length in silicon rapidly increases, and other materials must be used in the manufacture of a photodiode.
Recently, a most successful device capable of achieving high sensitivity at a longer wavelength (between 0.9-1.65 micrometers) has been an APD having separate absorption and multiplication regions, known as a SAM-APD (separate absorption and multiplication-avalanche photodiode). This device was first successfully demonstrated using InGaAs/InP (indium-galium-arsenide/indium phosphide) heterostructures sensitive towavelengths as long as 1.65 micrometers in a mesa geometry in 1981.
Unfortunately, mesa APDs have proven to be unreliable for use in practical systems applications, and it has become necessary to investigate the possible planar geometry structures which might be compatible with the requirements of the SAM-APD device. The basic design concept of the SAM-APD is that the electric field in the wide band gap InP multiplication region be large enough to induce carrier multiplication (and therefore photo current gain) while the field in the InGaAs absorbing region be low enough so that no tunnelling leakage currents are induced. These limits place stringent requirements on the epitaxial material layer thickness and dopings, particularly for the InP layer. In fact, the tolerances for these parameters are so tight, that normal planar geometries containing guard rings at the edges of the P-N junction which act to eliminate catastrophic edge breakdown effects are difficult to employ. Examples of such planar geometry attempts at producing heterojunction avalanche photodiodes include U.S. Pat. Nos. 4,651,187 (dated Mar. 17, 1987 to Sugimoto) and U.S. Pat. No. 4,684,969 (dated Aug. 4, 1987 to Taguchi).
Successful planar structures have been demonstrated which avoid the guard ring problem by utilizing highly complex regrowth structure. One such example may be found in U.S. Pat. No. 4,656,494 to Kobayashi (Apr. 7, 1987). Kobayashi discloses a buried structure avalanche multiplication photodiode having a surface level between the multiplication region and the guard ring region. Essentially, the structure disclosed in Kobayashi consists of absorption layers followed by the multiplication region. Next, annular areas which will eventually contain the guard rings are etched away leaving a groove, such as mesa 31 of FIG. 6(a) of Kobayashi which penetrates deep into the absorbing region. A low concentration end conductivity type layer 4 is selectively grown on the surface of the removed portion of the n-InP layer 3, shown in FIG. 5(e). The resulting structure has a flat parallel surface to the absorbing layer 2. This structure is complex to fabricate and may result in a potentially highly defected regrowth interface in the highest electric field regions of the device. Large leakage currents may result from such a structure except when material growth and fabrication conditions are held under the tightest possible control. Such devices are currently available in limited quantities and at a high price.
Another example of an avalanche photodiode which seeks to improve performance by preventing edge breakdown and the multiplication of the surface leakage currents in the junction periphery is U.S. Pat. No. 4,700,209 issued to Webb (Oct. 13, 1987). The Webb patent introduces a central zone 32a which has n-type conductivity and contains an excess areal concentration of conductivity modifiers above the background concentration. The central zone 32a is always of lesser extent than the cap region 36. The central zone is spaced apart from the absorption and cap regions where active multiplication occurs.
By enhancing the activity within the central multiplication region of the avalanche photodiode, while preventing edge breakdown, an optical fiber system may be designed having large repeater spacing for large capacity data transmission. This factor alone would bring a substantial system cost reduction. When future systems with improved avalanche photodiodes can be built operating in the region of 1.2 to 1.6 micrometers, a marked increase in repeater spacing is expected. Also a total fiber loss of 0.2 dB/km is achieved. Such results indicate that long wavelength APD detectors will allow a fiber optic system to achieve a repeater spacing greater than 60 km at 1 gigabit per second using a single mode fiber. Repeater spacing greater than 200 km may be achieved where the laser operates at 1.55 micrometers.
Therefore it is an object of this invention to provide a SAM-APD having high sensitivity and high reliability, but which is also easy to fabricate so that the final structure has low leakage currents with little tunneling or edge breakdown effects.